The photomultiplier tube (PMT) has dominated the area of single photon counting and single photon detection for many years. While possessing the desired photon counting functionality, i.e., outputting an explicit pulse upon reception of one single photon, a PMT is substantially bulky because of its vacuum tube nature. Besides, wavelength coverage and quantum efficiency of a PMT are both low. These drawbacks, plus low counting rate, high voltage and limited lifetime, make it impossible or very difficult to employ a PMT in many advanced areas, such as high speed photon counting, photon counting imaging, large format and spatially resolved photon and article detection, and space missions.
Geiger mode avalanche photodiodes (GAPDs) have also been employed for single photon counting. However, two major drawbacks, i.e., low speed/low counting rate and a requirement for external quenching, have severely inhibited advanced applications using Geiger mode avalanche photodiodes.
In particular, afterpulsing is an effect that substantially restrains the photon counting rate of a GAPD. Afterpulsing refers to a series of avalanche events triggered by the carriers that were generated in previous avalanche events, captured by the traps located at the vicinity of the avalanche region and then liberated again by these traps. The probability for afterpulsing to occur after detection of a signal photon is correlated to the amount of the traps located in the high field region and also the excess voltage, i.e., the amount of the bias above the breakdown voltage.
The time period needed for the afterpulsing probability to drop to below an acceptable level, which is known as the deadtime of the photon counting detector, normally ranges between microseconds and milliseconds. In order to prevent the afterpulses from being erroneously counted, photon counting should be blocked for a duration, which is named as reset time, after detection of a signal photon.
The upper portion of FIG. 1 shows a sequence 102 of photons incident into a gated GAPD. The lower portion of FIG. 1 shows the photon counting output 104 as a consequence of the photon events counted 106. Since the afterpulsing effect has high probability to occur in a certain time period or deadtime 108 after detection of a photon 110, photon incidency events 112, 114, 116 during the deadtime period are blocked or gated from counting, see reset time 118. As a result, the repeating rate of the photon counting is restricted by the magnitude of the deadtime.
Low avalanche initiation probability (AIP), which is general in GAPDs, leads to low quantum efficiency because the effective quantum efficiency is the product of AIP and the inner quantum efficiency. Avalanche initiation probability refers to the probability of a photoelectron to trigger an avalanche event in a GAPD detector. High avalanche initiation probability needs ideally uniform junction and multiplication region in the GAPD. Any artifacts, which include edge effect, defects and junction curvature, will lead to an “early breakdown” at a specific location and hence significantly reduce the probability for photogenerated carriers at other locations to trigger avalanche events there, leading to low AIP. Moreover, a GAPD pixel in an imaging array will suffer from a more severe AIP problem because its structure has to compromise with the processing technology.
More importantly, a Geiger mode avalanche photodiode (GAPD) requires an external quenching circuit to stop each of the avalanche events occurred inside it for otherwise the device will be burned out by the avalanching current. Because a GAPD pixel will become inactive after capture of one photon and the operation to reset the inactive pixels through the external quenching circuits is slow, a GAPD imager cannot recognize a scene unless the photon flux is very low, Leading to a very low dynamic range.
Passive and active quenching are the two quenching techniques currently available. Their typical circuits are schematically illustrated in FIG. 2 and FIG. 3. As discussed below, passive quenching is intrinsically slow because of the RC effect originated from the required high value of the quenching resistor. On the other hand, in view of chip area and power consumption requirements, active quenching does not suit array applications.
In particular, FIG. 2 shows a traditional Geiger mode Avalanche photodiode (GAPD) 202 in series with a passive quenching resistor 204. Once an avalanche effect occurs in the GAPD device 202, the high value of the resistor 204 will lead to a drop of the voltage across the GAPD 202 to below its breakdown voltage, hence quenching the avalanche process. A possible value for resistor 204 is 100 kΩ. Such high value of the resistor 204 results in a slow quenching process and even slower reset process, as also illustrated by the output waveform 206.
FIG. 3 shows a GAPD 302 with an active quenching circuit 304. While high speed can be achieved with the help of the quenching circuit 304, additional power consumption and space (or chip area, in the case of integrated circuits) will be required to implement this measure. For this reason, active quenching does not suit a GAPD imaging array. To exclude an erroneous counting of pulses resulting from the afterpulsing effect, a gating function 306 is incorporated in the quenching circuit 304 to prevent photon counting for a certain period after a real photon count. This, however, means that the afterpulsing effect will degrade the speed performance of a GAPD, even in the presence of a high speed quenching circuit.
There have been efforts to construct a “solid-state photomultiplier” (Christopher Stapels et al. “Solid-State Photomultiplier in CMOS Technology for Gamma-Ray Detection and Imaging Applications” Nuclear Science Symposium Conference Record, 2005 IEEE Volume 5, 23-29 Oct. 2005 Page(s): 2775-2779). However, such photomultiplier is an integrated detector array with one traditional GAPD and one passive quenching resistor at each pixel.
So-called “reach-through” structures are also known in the art. FIG. 4 shows the structure (upper portion of the figure) and the electric field bias profiles at different levels (lower portion of the figure) of such reach-through structure. This structure has been widely used in various avalanche photodiodes built on different semiconductor materials.
By inserting a layer 404 of p type reach-through structure beneath a multiplication region 406 in a structure also comprising a cathode 408, an anode 410, and an absorption region 412, high field will be restricted in the multiplication region 406 sandwiched between the cathode 408 and the reach-through structure 404. This will eliminate the probability for an avalanche event to occur outside the multiplication region 406. As the bias increases, the reach-through structure 404 will be fully depleted first and then the depletion region will penetrate into the absorption region 412, followed by breakdown occurring in the multiplication region 406.
The term “nanoscale” is used throughout the present disclosure to describe the lateral structural feature of all the component parts of the device according to the present disclosure that are contained in a pillar structure. In order to reflect the specific dimensional feature of the detector, the implication of the term “nanoscale” through the present disclosure is a little bit different from its usual meaning. In particular, throughout the present disclosure the term “nanoscale” will refer to a dimension ranging from 1-2,000 nm, and a nano-pillar will mean a pillar structure with a “nanoscale” diameter.